I didn’t have the ESM package from Ubuntu when I upgraded and it worked okay, so that isn’t the problem. Did you try to just curl that server from your command line and see if it works?
curl https://ppa.launchpadcontent.net/duplicity-team/duplicity-release-git/ubuntu/
Does all other connectivity work correctly? I wonder if Ubuntu was throttling or blocking your IP block because another customer of that provider was misbehaving?